This invention relates to a direct valve driving device for opening a valve by directly pushing it down with a rotating cam.
In a direct type valve driving mechanism, an intake or an exhaust valve is directly pushed down by a rotating cam. Such a direct type valve driving mechanism typically includes a valve lifter mounted between the cam and the top end of a valve stem. The force of a valve spring is applied to the valve stem to press the valve lifter against the cam. The valve stem can thus be pushed down by the cam through the valve lifter kept in contact with the cam.
A valve lifter of this type is disclosed in unexamined Japanese utility model publication 3-8603. This valve lifter has a nut member in abutment with the bottom surface of a closure end plate of a guide cap mounted between a cam and a valve. An engaging plate is mounted to the bottom end of an adjuster bolt threading into the nut member. The force of a return spring is applied to the engaging plate to apply torque in the protruding direction to the adjuster bolt, thereby keeping its bottom end pressed against the valve stem.
In a valve driving system with a valve lifter of this type, if a positive valve clearance forms between the cam and the valve lifter due to a difference in thermal expansion coefficient between the cylinder head, typically made of an aluminum alloy, and the iron valve, the adjuster bolt moves toward the valve stem while rotating under the force of the return spring, thus absorbing any change in the valve clearance. The valve can thus be opened and closed with extremely high reliability.
When the engine is cut at high temperature and cools down gradually, a negative valve clearance may form between the base circle of the cam and the valve lifter due to a difference in thermal expansion coefficient between the cylinder head and the valve. If this happens, the valve stem of the valve will be pushed by the base circle of the cam, thus opening the valve.
Ordinarily, an axial gap is present between the threads of the nut member and the adjuster bolt. If the axial gap is large enough, this gap serves to absorb any change in the valve clearance. But in conventional valve lifters, the size of the axial gap is not specified, so that the valve may open after the engine has been cut.
If the engine is started with its valves open, the pressure in the cylinders will not rise during the combustion stroke. It is thus desired to more accurately open and close valves.
In a direct valve driving device, the valve lifter is brought into contact with the cam by the force of the valve spring which is applied to the valve stem. Thus, while the intake or exhaust port is closed by the valve, the force of the valve spring is not applied to the valve lifter. Also, due to limited mounting space, the force of the valve spring is limited. Thus, a gap is formed between contact surfaces of the base circle of the cam and the valve lifter because the valve lifter cannot follow the rotation of the cam. Thus, the valve cannot be opened and closed with accuracy.
With a valve lifter, the adjuster bolt is threaded into the nut member abutting the bottom face of the closure end plate of the guide cap. The length from the cam contact surface of the guide cap to the bottom end of the adjuster bolt is thus long. The axial length of the valve driving device is thus long. That is, such a valve driving device is not compact enough.
Also, since the guide cap is lowered with the cam kept in contact with the closure end plate of the guide cap, the entire guide cap has to be made from a hard material. The nut member also has to be made from a hard material because it has to be wear-resistant. The entire weight is thus heavy. That is, the entire device is not lightweight enough.
An object of this invention is to provide a direct valve drive device which can open and close a valve with accuracy even if a negative valve clearance is present between the cam and the valve stem due to a difference in thermal expansion coefficient between the cylinder head made of aluminum alloy and the iron valve.
A second object of this invention is to provide a direct valve drive device which can drive a valve with accuracy.
A third object of this invention is to provide a compact and lightweight valve lifter which can drive a valve with accuracy.